Geochemistry of Darwin Glass and Target Rocks from Darwin Crater, Tasmania, Australia
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Meteoritics & Planetary Science 43, Nr 3, 479–496 (2008) AUTHOR’S Abstract available online at http://meteoritics.org PROOF Geochemistry of Darwin glass and target rocks from Darwin crater, Tasmania, Australia Kieren T. HOWARD School of Earth Sciences, University of Tasmania, Hobart, Tasmania 7000, Australia Present address: Impacts and Astromaterials Research Centre, Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK Corresponding author. E-mail: [email protected] (Received 1 March 2007; revision accepted 25 July 2007) Abstract–Darwin glass formed about 800,000 years ago in western Tasmania, Australia. Target rocks at Darwin crater are quartzites and slates (Siluro-Devonian, Eldon Group). Analyses show 2 groups of glass, Average group 1 is composed of: SiO2 (85%), Al2O3 (7.3%), TiO2 (0.05%), FeO (2.2%), MgO (0.9%), and K2O (1.8%). Group 2 has lower average SiO2 (81.1%) and higher average Al2O3 (8.2%). Group 2 is enriched in FeO (+1.5%), MgO (+1.3%) and Ni, Co, and Cr. Average Ni (416 ppm), Co (31 ppm), and Cr (162 ppm) in group 2 are beyond the range of sedimentary rocks. Glass and target rocks have concordant REE patterns (La/Lu = 5.9–10; Eu/Eu* = 0.55–0.65) and overlapping trace element abundances. 87Sr/86Sr ratios for the glasses (0.80778–0.81605) fall in the range (0.76481–1.1212) defined by the rock samples. ε-Nd results range from −13.57 to −15.86. Nd model ages range from 1.2–1.9 Ga (CHUR) and the glasses (1.2–1.5 Ga) fall within the range defined by the target samples. The 87Sr/86Sr versus 87Rb/86Sr regression age (411 ± 42 Ma) and initial ratio (0.725 ± 0.016), and the initial 43Nd/144Nd ratio (0.51153 ± 0.00011) and regression age (451 ± 140 Ma) indicate that the glasses have an inherited isotopic signal from the target rocks at Darwin crater. Mixing models using target rock compositions successfully model the glass for all elements except FeO, MgO, Ni, Co, and Cr in group 2. Mixing models using terrestrial ultramafic rocks fail to match the glass compositions and these enrichments may be related to the projectile. INTRODUCTION expected in small simple impact craters formed in sedimentary target rocks (Howard and Haines 2007). Glass Darwin glass is a siliceous impact glass found in a strewn distribution data are consistent with ejection from Darwin field covering more than 400 km2 in western Tasmania, crater and show: 1) the largest recovered fragments are found Australia. The glass also appears across the state in middens closest to the crater, 2) a decrease in the abundance glass and caves formerly visited by Tasmanian Aboriginals who are fragments away from the crater, and 3) an increase in the the traditional owners of the glass. Darwin glass has been proportion of splashform, relative to irregular or ropy, shapes repeatedly dated at ~800 ka (Gentner et al. 1973) and our away from the crater (Howard and Haines 2003; Howard most recent age estimate of 816 ± 7 ka was determined by Ar- 2004). Ar methods (Loh et al. 2002). Ford (1972; his Fig. 1) Impact glasses and tektites inherit the isotopic and identified a small (diameter = 1.2 km) circular depression geochemical signals of the source rocks melted during their near to the eastern edge of the strewn field in the Southwest formation. The aim of this paper is to test the compatibility of World Heritage Area (42°18.39′S, 145°39.41′E). This the suspected target rocks at Darwin crater as the materials pronounced topographic feature was named Darwin crater melted under impact conditions to form Darwin glass. Most and assumed to be the source of Darwin glass, despite a lack impact glasses and tektites have a composition similar to of conclusive evidence and a paucity of data on the glass or average upper continental crust and, as such, major- and trace- crater. The origin of Darwin glass has been investigated in a element composition alone is rarely enough to conclusively Ph.D. thesis (Howard 2004) and this paper stems from that relate a glass to a suspected target rock or to define an impact work. The geology of Darwin crater has been described in a origin. However, such investigations can effectively rule a detailed petrographic study and is consistent with that suspected target rock in or out of further consideration in 479 © The Meteoritical Society, 2008. Printed in USA. 480 K. T. Howard Fig. 1. Darwin crater and Darwin glass strewn field. The grey shaded area defines the Darwin glass strewn field. studies aimed at defining the impact origin of a glass. In this quartzite the west. Rocks at the crater are correlates of the paper, the geochemical and Sr,Nd isotopic systematics in Eldon Group; a succession of low-grade metasedimentary Darwin glass are shown to be compatible with the suspected rocks consisting of quartzites and slates (Fig. 3). Gould (1866) target rocks at Darwin crater. first named these rocks the “Eldon Beds” and defined them as the rocks overlying the main limestone succession (Gordon STRATIGRAPHY Group), near the mouth of the Gordon and along the Eldon Rivers. Gill and Banks (1950) formally defined the Eldon Darwin crater is situated on the Queenstown 1:250,000 Group and the following formations are recognized: (top) Bell scale geological map sheet (Corbett and Brown 1975; Fig. 2), Shale; Florence Sandstone; Keel Quartzite; Amber Slate and which shows significant geological complexity across the area Crotty Quartzite (base, Fig. 3). The metamorphic grade of the of the strewn field. The densely rainforested valley where rocks is lower greenschist, with the development of aligned Darwin crater lies is a tributary of the Andrew River valley. The micas that define the foliation cleavage of the slate units. In Andrew River valley cuts into Ordovician limestone (Gordon some samples, minor (<2%) chlorite alteration is also observed. Group). Siliceous conglomerate (Denison Group), Two drill holes penetrate the center of the crater. The unconformably dipping off Proterozoic quartzite of the deepest hole penetrated to a depth of ~230 m. Drill cores Engineer Range, forms the east side of the valley, and Silurian intersected fine-grained lacustrine sediments (~60 m thick), Geochemistry of Darwin glass and target rocks from Darwin crater 481 Fig. 2. Geology of the Darwin glass strewn field. Based on Corbett and Brown (1975) and Corbett et al. (1993). overlying poorly sorted coarser crater-fill deposits. The pre- crater-fill samples is far greater than in rocks surrounding the lacustrine crater-fill comprises a complicated package of crater but diagnostic shock indicators (e.g., planar deformation angular polymict breccias of quartz and country rock with very features [PDFs] in quartz grains) have not been observed. The rare glass, monomict sandy breccias of angular quartz, and petrographic features of the suspected target rocks and crater-fill clasts of deformed slate. The degree of deformation evident in samples are described in detail in Howard and Haines (2007). 482 K. T. Howard Fig. 3. Darwin crater geology. Based on field mapping in this study and by R. J. Ford; 1:25,000 scale aerial photographs; Corbett and Brown (1975) and Corbett et al. (1993). METHODOLOGY and these were mounted in epoxy discs 25 mm in diameter. In addition, 26 glass samples <5 mm in size and showing Scanning Electron Microscopy (SEM) splashform (droplet, spheroid, and elongate) shapes (Fig. 4) were collected from across the strewn field, cleaned, and Small pieces of Darwin glass were carefully chipped mounted intact in 25 mm epoxy discs. These “mini-glass” from individual larger fragments and placed in an ultrasonic grains were polished carefully to expose a fresh interior bath in distilled water for cleaning. The studied glass surface for analysis. This is the first time such splashform fragments were collected from across the strewn field. The mini-glass samples of Darwin glass have been reported or detached pieces’ sizes ranged from 2 to 10 mm in diameter analyzed. Geochemistry of Darwin glass and target rocks from Darwin crater 483 612, and the standard BCR-2 was analyzed as an unknown. During the analyses, two analyses of both NIST 612 and BCR-2 were made before ten analyses of Darwin glass, followed by two more analyses each of NIST 612 and BCR-2. As an internal standard the measured intensity of 49Ti during each analysis was normalized to the TiO2 content of each glass determined previously by SEM. Throughout the course of the study, detection limits were between 0.1 and 0.001 ppm. Spectra for data presented here are flat and smooth with little deviation in counts per second throughout the 60 s duration of each analysis. This is generally the case in analyses of tektites and impact glasses that are well suited to the LA-ICPMS technique, especially for determination of refractory elements (rare earth elements [REE], Sr). Suspected Target Rock Sample Selection and Preparation Fig. 4. Splashform Darwin mini-glasses. Scale bar = 1 cm. In total, 33 samples that encompass the range of Eldon Group rocks found around the crater and throughout the drill Major elements were determined using a CAMECA cores were selected for analysis. Most of the analyzed surface SX50 scanning electron microscope, in WDS mode, at the samples were collected along a W-E traverse from the access Central Science Laboratory (CSL), University of Tasmania. track, to the eastern edge of the valley hosting the buried The regulated electron beam current was operated at 25 nA at crater. These samples were collected in situ within the dense an accelerating voltage of 15 kV.